1. Field of the Invention
The present invention relates to a position measuring device for position measurement and to a method for position measurement.
2. Description of the Related Art
In many fields, absolute position measuring devices are increasingly being used. In such devices, the absolute position datum is derived from a code track having code elements disposed in series with one another in the measurement direction. The code elements are provided in pseudo-random distribution, so that a certain number of successive code elements each form one bit pattern. When the scanning device is shifted by a single code element relative to the code track, a new bit pattern is already formed, and by way of the entire measurement range to be detected absolutely, a sequence of different bit patterns is available.
One such sequential code is called a chain code or pseudo-random code.
The resolution of the absolute position generated with the absolute code is not precise enough for many applications. To further enhance the resolution, it is provided that the absolute position datum be combined with a position datum from a higher-resolution incremental position measurement. However, this combination can be made with certainty only if the resolution of the absolute position datum can unequivocally define at least one division period or incremental position measurement. To achieve the requisite total resolution, besides the absolute code, a plurality of incremental divisions of different division periods are therefore provided, as shown in German Patent Disclosure DE 4123722 A1. One of these incremental divisions has a division period corresponding to the width of one code element. By interpolation of the analog periodic scanning signal derived from this and attaching it to the absolute position datum of the code and by attachment to the high-resolution incremental division, a resultant absolute datum having the resolution corresponding to the high-resolution incremental division is obtained.
Proposals already exist of deriving an incremental position datum from the absolute code itself. For instance, in European Patent Disclosure EP 1329696 A1, an absolute code is disclosed with which on the one hand an absolute position datum as well as a plurality of analog incremental scanning signals, phase-shifted from one another, can be generated. Each code element comprises two partial fields, and one of these partial fields is disclosed periodically. For unequivocal absolute determination, the other of the two partial fields must therefore unequivocally encode the code element, and therefore the code overall includes three different partial fields. For generating the analog periodic scanning signals, in-phase scanning signals from a plurality of code elements are each added together.
This has the disadvantage that because of the periodic arrangement of partial fields, a third state is required for encoding the code elements.
In International Patent Disclosure WO 02/01160 A1, which is a point of departure for the present invention, a position measuring device is described in which the incremental position datum is derived from a code that includes a sequence of code elements of equal width, disposed in series with one another in the measurement direction, and having a first and a second property. The code elements are each disposed aperiodically. The incremental position datum is ascertained by Fourier analysis.
It is disadvantageous that the Fourier analysis in turn requires a periodic arrangement of the code elements. Moreover, relatively high computing power is necessary, and real-time processing is hard to achieve.